The microminiature vacuum tube utilizes electrons traveling in vacuum, and unlike the general vacuum tubes, it is formed on a semiconductor substrate. Therefore, a cathode of electric field emission type is used which emits electrons by means of an electric field. To emit electrons in such a cathode, the shape of the electron emitting end of the cathode is required to be as sharp as possible.
A description is given of an example of the conventional method of manufacturing a microminiature vacuum tube with reference to FIGS. 3(a)-3(e).
First, as shown in FIG. 3(a), a mask material is formed on the entire surface of a monocrystalline substrate 1, and the mask material on portions other than a portion 2 to become a cathode is removed by photolithography.
Next, as shown in FIG. 3(b), the substrate 1 is etched by dry etching such as RIE (reactive ion etching) using the mask material 2 as a mask. Furthermore, the substrate i is etched in the lateral direction and obliquely by anisotropic wet etching using an etchant such as potassium hydroxide, and a protrusion is formed which has an acute-angled tip 9 which becomes a cathode later (FIG. 3(c)).
Next, an insulating material 5 for protecting the tip shape of the cathode is formed on the entire surface of the substrate and a metal film 68 is formed thereon, and thereafter resist patterns 11 are produced thereon by photolithography (FIG. 3(d)). The metal film 68 and the insulating material 5 are etched by RIE or the like using resist patterns 11 as a mask and a gate 6 and an anode 8 are at periphery of the cathode formed on the substrate 1, thereby completing a device (FIG. 3(e)).
When this device is used, the cathode voltage Vc is made the ground level by grounding the substrate 1 as shown in FIG. 4, and a voltage V.sub.A of 100 to 500 V is applied to the anode 8. Electrons emitted from the cathode 9 into vacuum by means of electric field emission are collected by the anode 8. Meanwhile, the quantity of electrons flowing from the cathode 9 to the anode 8 is controlled by applying a voltage of several tens of volts to the gate 6 as a gate voltage V.sub.G.
In the conventional microminiature vacuum tube manufactured by the method as described above, etching in the lateral direction is utilized to form the cathode, therefore the control of timing for ending etching when the tip shape of the cathode becomes acute-angled is very difficult. Particularly, in fabricating a plurality of cathodes on the substrate, this control is further difficult. Actually, as shown in FIG. 5, a cathode 12b which has not been etched fully, a cathode 12c which has been etched excessively and the like are formed besides a cathode 12a having a desired shape. Thus, variations occur in the shape of the cathode.
Also, the area of adhesion between the portion to become the cathode on the surface of the substrate 1 and the mask material 2 becomes smaller as the etching progresses, and therefore the adhesion force between the both is weakened. This results in peeling of the mask material and the etched shape varies. Therefore it is difficult to obtain a uniform etched shape.
Further, the tip of the cathode is required to be protected when the gate and the anode are formed, and in the conventional example, the tip is protected by an insulator film such as SiO.sub.2. However, the tip part of the cathode is actually exposed to an etching gas immediately before the gate 6 and the anode 8 are formed, and for this reason, the tip part of the cathode is damaged and it is difficult to maintain the original sharp tip shape.
As described above, in the conventional manufacturing method, the controllability and the reproducibility of the etching process for forming the cathode are worse, and further the tip part of the cathode is damaged in the stage of forming the gate and the anode, incurring non-uniformity in the device characteristics.